FX.49 BimCraft: Add levels takes heights below the lowest level

Asked on Discord by a Discord member: is there a way to create a level below the lowest level made so far? The Levels panel already had Add a level below (DC.119). The Add levels dialog (At these heights) still refused anything under 0; it now takes negative heights (for example -10'-0", -20'-0"): the new level reaches up to the next level above it and is named Level -1 (nearest the ground), Level -2, and so on. One undo step. Test: draw06 'Add levels at -6000 and -3000' (225/0). Not tested: a real project with walls or a slab at the lowest level (the levels are added, nothing else moves), a desktop build.